1. Field of the Invention
The present invention relates to a semiconductor device using a thin film transistor (hereinafter referred to as a TFT) wherein a semiconductor film formed on a substrate and having a crystal structure is used; and a process for manufacturing the same. The semiconductor device in the present specification means any device which functions by the use of a semiconductor characteristic, and the category of the semiconductor device manufactured by the present invention includes a display device, a typical example of which is a liquid crystal display device having therein TFTs, and a semiconductor integrated circuit (such as a microprocessor, a signal processing circuit, or a high frequency circuit).
2. Description of Related Art
In various semiconductor devices having therein semiconductor elements, such as a television, a personal computer or a portable telephone, a display for displaying characters or images is an essential means from which people recognize information. Particularly in recent years, a planar display (flat panel display), a typical example of which is a liquid crystal display device using the electrooptic property of liquid crystal, has been actively used.
As one form of the flat panel display, there is known an active matrix driving system wherein a TFT is fitted to each pixel and a picture is displayed by writing data signals successively. TFTs are essential elements for realizing the active matrix driving system.
In almost all cases, TFTs are manufactured using amorphous silicon. However, TFTs have a low electric field effect mobility and cannot be operated by frequencies necessary for processing picture signals. Therefore, TFTs are used only as switching elements fitted to respective pixels. A data line driving circuit for outputting picture signals to data lines or a scanning line driving circuit for outputting scanning signals to scanning lines is processed by an outside IC (IC driver) mounted by TAB (tape automated bonding) or COG (chip on glass).
However, as the density of pixels becomes larger, the pitch of the pixels becomes narrower; therefore, it is considered that the system in which a driver IC is mounted has a limit. For example, in the case that UXGA (pixel number: 1200×1600) is supposed, 6000 connecting terminals are required in an RGB coloring system at the simplest estimate. An increase in the number of the connecting terminals causes an increase in the probability that contact faults are generated. Moreover, the peripheral area (frame area) of its pixel section increases. As a result, it is unsuccessful that a semiconductor device using this as a display is made small-sized, and the design of the appearance thereof is damaged. In light of such background, a display device integrated with a driving circuit is clearly demanded. By integrating a pixel section with scanning line and data line driving circuits on a single substrate, the number of connecting terminals decreases drastically and the area of the frame area can also be made small.
As a means for realizing the above, suggested is a method of making TFTs of a polycrystal silicon film. However, even if polycrystal silicon is used to form TFTs, after all the electric properties thereof are not equivalent to the properties of MOS transistors formed in a monocrystal silicon substrate. For example, the electric field effect mobility thereof is not more than 1/10 of that of monocrystal silicon. This method has a problem that off-state current is large because of defects formed in boundaries between crystal grains.
However, for a data line driving circuit, high driving ability (on-state current, Ion) is required and an improvement in the reliability thereof is also required by preventing deterioration based on hot carrier effect. In addition, for a pixel section, low off-state current (Ioff) is required.
As a TFT construction for reducing the off-state current, a lightly doped drain (LDD) construction is known. This construction is a construction wherein an LDD region, to which an impurity element is added at a low concentration, is arranged between a channel formed region and a source region or a drain region formed by the addition of an impurity element at a high concentration. As a construction effective for preventing deterioration of the on-state current based on hot carries, there is known an LDD construction wherein a gate element is overlapped with some portions of an LDD area (hereinafter, the gate-drain overlapped LDD is abbreviated to the GOLD).
In pixels in an active matrix driving system provided with TFTs, pixel electrodes for applying voltage to liquid crystal are set up, and scanning lines (gate lines) connected to gate electrodes cross data lines connected to sources or drains. As an auxiliary capacitor, the following two types are known: an addition capacitor type, wherein a pixel electrode is overlapped with a scanning line (gate line) in the preceding stage; and an accumulation capacitor type, wherein an exclusive capacitor line is set up. In either case, as image quality is made finer and minuter, the size of the TFT or the auxiliary capacitor allowable for each pixel is inevitably made smaller. Therefore, in order to make the numerical aperture of each pixel high within a restricted scope of pixel size, it is indispensable to lay out elements necessary for the construction of these pixels effectively.
TFTs are manufactured by laminating semiconductor films, insulating films, or conductive films while etching them into given shapes, using photo masks. However, if the number of the photo masks is simply increased in order to make the construction of the TFTs optimal in accordance with requests for a pixel section or respective driving circuits, the manufacturing steps become complicated and the number of the steps increases unavoidably.
In order to realize a high numerical aperture within a restricted scope of pixel size, it is indispensable to lay out elements necessary for the construction of a pixel section effectively.